The present invention relates to a static drop mercury electrode and more particularly to a valve control for the formation of the static drops.
Polarography, one of a broad class of voltametric techniques, provides chemical analysis of substances in electrolyte solution by the observation of current and voltage relationships at electrodes immersed in the solution. At low voltages no current flows between the electrodes. However, as the voltage is increased and becomes great enough for deposition of each reducible substance in solution on the electrodes, current begins to flow. Generally, the magnitude of the current is proportional to the concentration of the reducible substance in solution and the magnitude of the voltage potential required to induce this current flow is indicative of the density of the substance in solution. Thus, precise measurement of the current through the immersed electrodes, as a function of applied potential, provides both qualitative and quantitative analysis of the reducible substances in solution.
The electrical and chemical antithesis of polarography is the technique of stripping voltametry whereby the reducible substances in solution are concentrated by plating the substances onto an immersed electrode. Plating is accomplished by holding an immersed electrode at suitable potential. The applied potential is then varied in the anodic direction, with the plated electrode biased toward increasingly more positive values. The substances plated on the electrode oxidize into solution at potentials corresponding to the oxidation potentials of the plated substances. Again, current flow occurs with the oxidation of each substance and analysis of the substances is achieved in a manner analogous to the polarographic analysis described above.
Typical polarographic and stripping voltametry apparatus consists of a variable voltage source, a current measuring circuit and an electrolysis cell. The cell typically contains three electrodes immersed in the solution. The three electrodes comprise a reference electrode at which the variable potential is applied, a working or indicator electrode at which current flow is measured, and an auxiliary or counter electrode which regulates the potential between the reference and working electrodes.
The most widely used working or indicator electrode is the dropping mercury electrode, which consists of a fine bore capillary tube above which a constant head of mercury is maintained. The mercury emerges from the tip of the capillary at the rate of a few milligrams per second and forms spherical droplets which fall from the capillary orifice into the solution at a typical rate of one every two to ten seconds. The capillary comprising such a dropping mercury electrode must have a small enough bore so that the adhesion between a mercury drop and a column of mercury above the drop, the cohesion of the mercury column and the interfacial tension between the drop and the solution under test is sufficient to allow the drop to form. When the bore of the capillary exceeds approximately twelve hundredths of an inch, the drop no longer is able to hang at the orifice of the capillary. A suitable capillary comprises, for example, marine barometer tubing of three thousandths of an inch bore. Capillary of a fine bore type capable of establishing mercury drops is hereafter referred to as a "dropping mercury capillary."
The dropping mercury electrode has a number of advantages over other varieties of electrodes. First, mercury has a hydrogen over-voltage which allows observation of processes which would normally be obscured by the decomposition of water at other electrodes. In addition, periodic renewal of the surface area of the dropping mercury electrode minimizes problems due to surface composition changes.
However, since the inception of polarography, all voltametric measurements at a dropping mercury electrode have been complicated by the time dependence of the surface area of the growing mercury drop. More specifically, a double-layer charging current is produced by the growth of the mercury drop which varies as the drops grow and, hence, interferes with obtaining accurate polarographic measurements.
Prior art techniques have attempted to overcome the disadvantages of a dropping mercury electrode by the use of pulse polarographic techniques in connection with a drop kicker in an attempt to minimize the effects of drop growth during voltametric analysis by assuring analysis occurs during the same period of growth for each drop. Basically, the drop kicker of the prior art applies a periodic mechanical pulse to the capillary, disengaging the mercury drop hanging therefrom. The dislodgement of the drop serves as a starting point for the timing of a subsequently developed drop. A potential pulse is applied to the mercury drop a fixed time after activation of the drop kicker in an attempt to assure that the polarographic measurement is taken with a drop of given repeatable size. While the pulse polarographic technique employing a drop kicker represents an improvement over the standard dropping mercury electrode, nevertheless, during sampling, the mercury drop is increasing in size causing imprecise resultant measurements.
Another prior art technique establishes a stationary hanging mercury drop at the end of a capillary tube by selectively decreasing the height of the mercury column after formation of a drop to prevent additional drop growth. In this technique a single hanging drop is held stationary at the end of the capillary for a long time, on the order of thirty minutes. While this technique results in a constant area mercury drop, a single drop is required to be held stationary for so long that the surface of the drop is subject to composition change which interfers with the accuracy of the measurements obtained. Furthermore, high analysis resolution by rapid removal and replacement of drops and repetitive measurements cannot be achieved by this prior art technique.
Still another form of prior art mercury electrode employs a relatively large bore capillary formed with a U-shape adjacent its lower end which allows the end of the capillary to support the resultant sessile drop. In this form of apparatus, it has been suggested to employ a form of sliding gate valve in the large bore capillary which may periodically be opened and closed to form constant area sessile drops supported on the end of the large bore U-shaped tubing. Such a system has little commercial or scientific value since the sessile drop of that system does not and cannot have the desired spherical geometry of a hanging mercury drop, is susceptible to causing solution contamination of the large bore capillary and is more difficult to dislodge than a hanging drop. Furthermore, hanging mercury drops cannot be formed by the employment of sliding gate valves since a hanging mercury drop requires employment of a dropping mercury capillary of fine bore and sliding gate valves for such fine bore capillaries inherently trap small volumes of air. Moreover, sliding gate valves of the prior art, even in large capillary systems, cannot adequately isolate the mercury column from air which, if introduced into the column in even the smallest of amounts, a fraction of a microliter, seriously interferes with the stability of a hanging mercury drop.
It must be understood, to appreciate the significance of even the smallest amount of air in a mercury column, that it acts as a spring in the system so that after a drop which has been hanging on the capillary tip falls, the pocket formed by the air contracts and the resultant pressure differential pulls the solution part way up the capillary, thereby contaminating the capillary. Furthermore, even the smallest amount of air may sporadically break loose and flow down the capillary causing breaks in the required electrical continuity of the capillary.
The problem of trapped air particularly affects an additional prior art mercury drop electrode employing a plunger delivery system for dispensing mercury drops. A typical prior art plunger delivery system consists of a dropping mercury capillary with an enlarged bore at its upper end. A plunger is fitted at this bore to form a seal with the bore. The bore is filled with mercury and the plunger is advanced down the bore and drops of mercury form at the capillary tip. As is true with sliding gate valves, the seal between the bore and the plunger inevitably introduces air into the mercury system since the high surface tension of mercury readily traps bubbles of air against the capillary bore and around the plunger seal. Another shortcoming of the plunger delivery system is that minute variations in the bore diameter or bore-to-plunger friction can materially affect the drop size as the plunger advances down the bore. Furthermore, such a system is costly in that it requires several precision parts including a motorized micrometer screw drive for advancing the plunger.
It should also be noted that in none of the above-mentioned prior art techniques can the dropping mercury capillary be readily removed from the electrode or inserted into the electrode without spillage of mercury.
It is accordingly an object of the present invention to provide a static mercury drop electrode for periodically generating static mercury drops of reproducible size at the end of a dropping mercury capillary.
It is another object of the present invention to provide a static mercury drop electrode for generating static mercury drops of predetermined size in which air is prevented from interfering with the operation of the capillary.
It is still another object of the present invention to provide a static memory drop electrode in which predetermined amounts of mercury are allowed to flow down a dropping mercury capillary, without the introduction of air into the capillary, to allow for the formation of static mercury drops of reproducible size at the lower end of the dropping mercury capillary.
Another object of the present invention is to provide a static mercury drop electrode in which a dropping mercury capillary is readily replaceable without the introduction of air into the mercury system.
A still further object of the present invention is to provide a mercury drop electrode which can readily be employed as a dropping mercury electrode, hanging mercury drop electrode and as a static drop mercury electrode without physical changeover.
Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.